A spark plug for use in an internal combustion engine such as an automobile engine includes a center electrode extending, for instance, in an axial direction thereof, an insulator disposed on an outside of the center electrode, a generally tubular metal shell disposed on an outside of the insulator, and a ground electrode having a base end portion that is connected to a tip end portion of the metal shell. The ground electrode is arranged in such a bent-back state that a tip end portion thereof is opposed to a tip end portion of the center electrode. With this arrangement of the ground electrode, a spark discharge gap is generated between the tip end portion of the center electrode and the tip end portion of the ground electrode. Further, there is known a spark plug of a conventional art in which a noble metal chip having a relatively small volume is provided on a portion of the ground electrode defining the spark discharge gap. With the provision of the noble metal chip, the spark plug of the conventional art aims to prevent the ground electrode from taking heat of a flame kernel (that is, quenching phenomenon) and thereby improve the ignition property.
In recent years, with increase in material cost for the noble metal chip which is caused due to lack of resource, there is an increasing demand for such a spark plug constituted without using the noble metal chip in the ground electrode. For this reason, in view of improving the ignition property even when the noble metal chip is not used in the ground electrode, there has been proposed a spark plug in which a portion of the ground electrode which is to be opposed to the center electrode is formed into such a trapezoidal shape in section as to project toward the tip end of the ground electrode, and opposed side faces of the tip end portion of the ground electrode are tapered toward the side of the tip end of the ground electrode (for instance, see Patent Literature 1). According to this conventional art, the quenching phenomenon can be suppressed so that increase in ignition property can be expected.
However, in the spark plug of the above-described conventional art, the ground electrode is not formed into the tapered shape except for the tip end portion thereof, that is, the ground electrode has a relatively large width at the bent portion and the root side portion. Therefore, in the case of the spark plug of the above-described conventional art, an abnormal spark discharge will readily occur between the tip end portion of the center electrode and the bent portion of the ground electrode similar to the ground electrode which is configured to have a substantially uniform width. This will fail to sufficiently attain the effect of improving the ignition property.
In order to solve the above problem, there has been proposed a spark plug including a ground electrode with a tip end portion which is tilted toward the side of a tip end surface of the center electrode such that a corner of the tip end of the ground electrode is opposed to the tip end surface of the center electrode (for instance, see Patent Literature 2). With this construction, occurrence of a spark discharge (i.e., a spark discharge at a normal position) between the corner of the tip end of the ground electrode and the center electrode can be facilitated to thereby increase the ignition property.
However, in a case where the spark plug of the above conventional art is used, increase in ignition property of the spark plug can be attained but local wastage of a portion of the center electrode which is opposed to the corner of the tip end of the ground electrode tends to proceed. As a result, rapid expansion of the spark discharge gap is caused, and therefore, failure to provide a normal spark discharge will occur at an early stage of the usage.
On the other hand, there has been proposed a spark plug including the ground electrode which has a curved face on at least a side opposed to the center electrode (for instance, see Patent Literature 3). With this construction, an air-fuel mixture can be readily flowed into the spark discharge gap to thereby increase the ignition property without deterioration in life of the spark plug.
Citation List
Patent Literatures
    Patent Literature 1: Japanese Patent No. 3461637    Patent Literature 2: Japanese Patent Application Unexamined Publication No. 2004-87464    Patent Literature 3: Japanese Patent Application Unexamined Publication No. 2007-250344